Touch sensors are transparent or opaque input devices for computers and other electronic systems. As the name suggests, touch sensors are activated by touch, either from a user's finger, or a stylus or some other device. Transparent touch sensors, and specifically touchscreens, are generally placed over display devices, such as cathode ray tube (CRT) monitors and liquid crystal displays, to create touch display systems. These systems are increasingly used in commercial applications such as restaurant order entry systems, industrial process control applications, interactive museum exhibits, public information kiosks, pagers, cellular phones, personal digital assistants, and video games.
The dominant touch technologies presently in use are resistive, capacitive, infrared, and acoustic technologies. Touchscreens incorporating these technologies have delivered high standards of performance at competitive prices. All are transparent devices that respond to a touch by transmitting the touch position coordinates to a host computer. Each has, of course, relative strengths and weaknesses.
Acoustic touchscreens, also known as ultrasonic touchscreens, have competed effectively with the other touch technologies. This is due in large part to the ability of acoustic touchscreens to handle demanding applications with high transparency and high resolution touch performance, while providing a durable touch surface. Acoustic touchscreen systems comprise a transparent touch sensor (i.e., a touchscreen), a controller and leads coupling the touchscreen and the controller. Typically, the touchscreen comprises a touch sensitive substrate in which an acoustic wave is propagated. When a touch occurs on the substrate surface, it results in the absorption of at least a portion of the wave energy being propagated across the substrate. The touch position is determined using electronic circuitry to locate the absorption position in an XY coordinate system that is conceptually and invisibly superimposed onto the touchscreen. In essence, this is accomplished by recording the time the wave is initially propagated and the time at which a touch induced attenuation in the amplitude of the wave occurs. The difference in these times can then be used, together with the known speed of the wave through the substrate, to determine the precise location of the touch.
A common type of acoustic touchscreen employs Rayleigh type acoustic waves—where the term is intended to include quasi-Rayleigh waves. Illustrative disclosures relevant to Rayleigh wave touchscreens include U.S. Pat. Nos. 4,642,423; 4,645,870; 4,700,176; 4,746,914; 4,791,416; Re 33,151; U.S. Pat. Nos. 4,825,212; 4,859,996; 4,880,665; 4,644,100; 5,739,479; 5,708,461; 5,854,450; 5,986,224; 6,091,406; 6,225,985; and 6,236,691. Acoustic touchscreens employing other types of acoustic waves such as Lamb or shear waves, or combinations of different types of acoustic waves (including combinations involving Rayleigh waves) are also known. Illustrative disclosures of these technologies include U.S. Pat. Nos. 5,591,945; 5,854,450; 5,072,427; 5,162,618; 5,177,327; 5,329,070; 5,573,077; 6,087,599; 5,260,521; and 5,856,820. The above cited patents are hereby incorporated by reference into this application.
Acoustic touchscreens, including Elo TouchSystems, Inc.'s IntelliTouch® products, which sense touch via the absorption of Rayleigh waves, have proved to be commercially successful. The success of products using Rayleigh waves is due in large part to two properties exhibited by Rayleigh waves. First, Rayleigh waves are typically more sensitive to touch than are other acoustic waves. Second, Rayleigh waves are surface waves that can propagate on the surface of any simple homogenous glass substrate of sufficient thickness.
However, Rayleigh waves are sensitive to liquid contaminants such as oil and water. These contaminants absorb energy from the propagating waves. A drop of water can generate a signal that in many ways behaves like a touch signal. Contamination of the touchscreen by water on the surface, say as the result of a spill, a sneeze, or rain, can produce false readings, since the contaminant absorbs some of the Rayleigh wave and consequently attenuates a portion of the wave amplitude, i.e., it generates a dip in the wave amplitude. It is important to minimize the adverse effects of water and other contaminants.
One approach that has been used to deal with the problem of water contamination, makes use of the fact that—depending on the amount of water contamination—the corresponding amplitude dip may be wider than for a normal finger touch. This phenomenon is illustrated in FIGS. 1(a)-(c), which shows three separate graphs of amplitude versus time. FIG. 1(a) shows the behavior with no touch or water, and hence shows no dip at all. FIG. 1(b) shows the dip associated with a finger touch. FIG. 1(c) shows the dip associated with water contamination. As FIGS. 1(1b) and 1(c) illustrate, the water contamination dip may be wider than the normal finger touch dip, and thus, such water contamination can therefore be distinguished from normal finger touches. This water rejection algorithm has been in use since the earliest commercial acoustic touchscreen products. A later more explicitly documented example of the application of algorithm use of dip widths (but not applied to water contamination) is found in U.S. Pat. No. 5,638,093, which considers the use of dip width to differentiate between different types of touches. This patent is hereby incorporated by reference into this patent application. Unfortunately, not all water drops produce wide dip shapes, and thus, it is often difficult to distinguish between water contamination and actual touches using this methodology.
Another ameliorative approach involves reference waveform updating. A static dip from a water contaminant is eventually “memorized out” and becomes part of the reference waveform. Thus, an implementing algorithm may redefine as a contaminant any dip that remains static for more than 30 seconds, for example, and update the reference waveform accordingly. Of course, an appropriate wait time, such as, for example, 30 seconds, must be carefully established. This is important because, like contaminants, valid touches may also have a significant duration. Such an updating algorithm prevents a valid touch from being blocked by a water drop that landed on the touch surface well in advance of the finger touch. This algorithm thus improves system performance.
However, there is still a problem with water contamination of width comparable to a finger, that arrives close in time to a valid finger touch. For example, if the user's fingers are wet from the condensation of a cold drink, a valid finger touch may also leave a residue of water contamination with a width the size of finger touch. In such cases, there is a risk that the ultrasonic touch system will continue to consider the wet finger to be touching the touchscreen surface, and thus, block out the next valid touch. One way to obviate this difficulty is simply to declare that the last touch to appear “wins.” Algorithms that use this approach have the advantage that a valid touch will automatically override any contaminant induced dips, so that the system will respond “instantaneously” to the user's latest input. Thus, even if water contaminants cause a spurious input, the user quickly regains control.
While reasonably effective in reducing the negative effects of water on system performance, the “last touch wins” algorithm does not support multiple simultaneous touches. Multiple touch algorithms are considered in U.S. Pat. No. 5,854,450, which is hereby incorporated by reference. Applications that demand multiple touch capability, such as, for example, two-player video games, still demand water rejection that matches or improves upon current levels of water-rejection performance. Present algorithms cannot satisfy both these demands. This increases the need for additional algorithm methods for recognizing and rejecting water contamination.
Different acoustic modes vary in their susceptibility to water. The Rayleigh waves typically used in ultrasonic touchscreens are strongly absorbed by radiation damping, even for a zero viscosity fluid. In contrast, absorption of shear waves depends only on viscous damping. As a result, shear waves are much less affected by water than are the more commonly used Rayleigh waves. For example, U.S. Pat. No. 5,177,327 teaches a variant of ultrasonic touchscreen technology using horizontally polarized shear waves. Touchscreens using this technology demonstrate a high degree of water immunity. Indeed, the substrate surface can be completely submerged in water, and still correctly sense the position of finger touches. For ultrasonic touchscreen applications with very high levels of water contamination, use of shear acoustic modes may well be required.
The use of shear acoustic modes, however, comes with a cost. Shear modes are generally much less sensitive to touch than are Rayleigh waves. Furthermore, the use of shear modes often require complex, and hence more costly, substrate designs than does the Rayleigh mode. For applications subject to more moderate levels of water contamination, it is therefore preferable, whenever possible, to enhance water rejection using signal processing, and to continue to use the lower cost Rayleigh mode touchscreens.
Hence, despite the shear acoustic mode option, there remains a significant need for improved water rejection signal processing that supports multiple touch capability. Moreover, even if multiple touch capability is not required, there is still a need for improved water rejection signal processing, since any improvement in water tolerance will enable ultrasonic touchscreens to be used in applications with higher levels of water contamination.